The expoxy resins have excellent adhesiveness as well-known and have been used for bonding a metal, a glass or a wooden product or a coating on the surfaces thereof. The epoxy resins have excellent heat stability, mechanical characteristics and electric insulation as well as excellent adhesiveness, bending property and chemical resistance whereby the epoxy resins have been used in various fields such as casted products, glass fiber laminated products and electric insulating products.
However, in general, the epoxy resins have relatively low flexibility whereby cracks may be easily formed in manufactures of large size impregnated products and large size casted products having complicated shapes and have brittle coated film in applications of non-solvent type varnishes, disadvantageously.
In order to improve the brittleness of the epoxy resin and to impart flexible characteristics, it has been known to blend a polyamide resin, an epoxide vegetable oil or plasticized epoxy resin.
However, when these materials are incorporated in the epoxy resin, the mechanical and electrical characteristics are remarkably deteriorated and the usages of the resulting products are disadvantageously limited.
It has been known that typical epoxy resin compositions for impregnation can be prepared by incorporating a hardener such as acid anhydride, e.g. methyl tetrahydrophthalic acid anhydride, a hardening accelerator such as tertiary amines, imidazoles, metal chelating compounds to a bis-phenol type epoxy resin.
These epoxy resin compositions have disadvantages of relatively short life and undesirable viscosity for processing.
It has been needed to obtain epoxy resin compositions which have superior characteristics for processing and long pot-life and form hardened products having excellent electrical and mechanical characteristics such as insulation.